The present invention relates to a method of removing solid material buildup from granular media filters and bioreactors. Particularly, the invention is directed to a method of removing solid material buildup from granular media filters and bioreactors utilizing an air and water backwash system.
Communities often build sewer systems that would collect both storm water runoff and sanitary sewage in the same pipe. Combined sewer overflows (CSO) result in overflows that contain not only storm water but also untreated human and industrial waste, toxic materials and debris. Storm waters can also infiltrate or leak into conventional wastewater collection systems. Typically, wastewater treated in granular media filters and bioreactors contain large amounts of solid material. During storm surges, however, the amount of suspended solids increases to a point greater than the capacity of the filter treatment plant. Sewer systems are presently designed to discharge this excess of polluted CSO wastewater directly into nearby streams, rivers, lakes or estuaries. See, Fleming and Slack, Trends in Sewer Overflow Management, Water Engineering and Management, February, 2001. During normal operations of wastewater treatment filters, solids collect on the filter media. The collection and buildup of layers of solids on the filter media prevents the wastewater from being in direct contact with the media and therefore reduces removal of solids and BOD from the wastewater. Also, the build-up of solids on the filter causes head loss. The solids form a layer on top of the filter as well as layers on the surfaces of the filter media. This results in head loss and the unit must be cleaned in order to restore optimal performance to the filter again. Air and water backwash is required to effectively remove these solids. During storm surges, wastewater load on treatment filters can increase, for example, from the normal 60 million gallons per day to 360 million per day during a storm surge. Storm surge flows can contain suspended solids up to 500 mg/l which is a 600% increase from normal conditions. Depending on the treatment facility, this overload of solids can shut down the filter.
Typically, the removal of solids from the filter system is discretely accomplished during a backwash process, meaning that a single filter is isolated from the treatment system and the backwash process for the removal of suspended solids is initiated and completed before the next filter is initiated into the process. Further, while a specific filter is removed from the system, remaining filters in the system must handle the additional workload of the isolated element. Several apparatus and methods of backwashing have been taught.
Bennick et al., U.S. Pat. No. 5,792,373 teaches a backwash filter array comprising a xe2x80x9cplurality of filter unitsxe2x80x9d each having a housing, a filter element, and inlets and outlets. The process liquid inlet is capable of being switched in order to be connected to either to the process liquid source or the backwash liquid drain. The xe2x80x98373xe2x80x99 reference discloses an arrangement where the valves can be arranged to allow filter units to backwash simultaneously, so that filtration capacity and backwash efficiency can be increased.
The Wyness xe2x80x98019xe2x80x99 reference discloses a method for backwashing filters in a water treatment plant with clarifier and peripheral filter cells. The reference discloses an apparatus for treating a liquid containing suspended solids comprised of a clarifying vessel and a number of filters positioned around the periphery of the clarifying vessel for receiving and filtering liquid from the clarifier. The xe2x80x98019xe2x80x99 reference teaches that a filter may be backwashed by isolating that filter, filling the other filter cells with clarified liquid and draining them into the isolated cell to remove filter media contaminants.
Hensley, U.S. Pat. No. 5,833,867 teaches a system and method for backwashing multiple filtration vessels. The Hensley xe2x80x98867xe2x80x99 reference discloses a system of multiple filtration vessels each of which utilizes a common decontaminating backwashing unit or system located on the exterior of the vessels. Each filtration vessel has a filtration screen dividing the vessel into a filter media containing chamber and a filter media free chamber. The decontamination unit is located inside of the filter media containing chamber. The xe2x80x98867xe2x80x99 reference also teaches the use of the same dirty process fluid to backwash the filtration media as used to produce the filtered water. Also, Hensley xe2x80x98867xe2x80x99 discloses the use of a circulation or contamination separating pump to dislodge contamination on the filter media particles.
None of the above-listed references discuss the problems associated with CSO and infiltration. CSO""s discharge contain a variety of pollutants that may adversely affect the receiving water body, including pathogenic microorganisms, viruses, cysts, chemicals and floatable materials. Because of the rapid buildup of solids in the filter during storm surges, traditional methods of backwashing cannot keep up with the surge of wastewater flow and the CSO must be discharged into nearby rivers and streams. Reducing CSO discharge by improved backwashing so that filters can handle greater loads is an object of this invention.
The present invention relates to an improved method for backwashing granular media treatment systems comprising multiple filters in a continuous cycle. The method of backwashing removes suspended solids from filter media and the filter infrastructure with a reduction of the backwash time required for each filter and a minimum of backwash water used in restoring flow capacity. This allows the filter system to operate at a time of very high solids/hydraulic loading in situations such as storm surges. Reducing the backwash time reduces the time each filter is out of operation so that treatment filters operate more efficiently and are better able to handle the combined sewer overflows produced by the storm surges. One innovative aspect of this backwash method is to reduce the typical air/water backwash time for each filter from about 35 minutes to about 5 minutes or less. To accomplish this reduction of backwash time, the method is characterized by a continuous cycle of backwashing that backwashes two filters simultaneously. Efficient backwashing requires two steps, a first air/water bashwash and a second water-only or rinse backwash. In the preferred method, as one filter is at the air/ water backwash stage, the backwash cycle of a second filter is started so that the second filter is air/water backwashed during the water-only backwashed stage of the previous filter. According to the method of this invention, each filter in a series of filters is out of the filtration process for only about 5 minutes, thereby minimizing downtime and disruption of the filtration process necessary to restore the capacity of the filters caused by buildup of solids. A seamless transition from one filter to another occurs during the backwash stages. This enables the filters to operate at a high level of solid/hydraulic loading. Also the continuous cycling of the backwash process through the filters in a system minimizes the start/stop action on the backwash water pumps and air blowers saving wear and tear on this equipment.
One preferred method for removal of solids in a water treatment system having multiple filters, each filter comprising an air backwash system and a water backwash system, comprises a series of steps to enable two filters to be in some stage of the backwash process at the same time thereby reducing the time the filter is out of service. Not all of the steps are sequential. The steps can comprise:
(a) closing influent valve and effluent valve of initial filter and opening backwash air valve of first or initial filter of series of filters in filter treatment system;
(b) turning on backwash air blowers for air backwash, start air backwash of first or initial filter;
(c) opening backwash water valve and dirty backwash water valve of initial filter;
(d) turning on backwash pumps for water backwash, start air/water backwash;
(e) backwashing initial filter with an air/water backwash;
(f) closing influent and effluent valves of next filter, opening backwash air valve of next filter;
(g) closing air backwash valve on initial filter, start air backwash on next filter;
(h) opening backwash water valve and dirty backwash water valve on next filter, begin air/water backwash of next filter while continuing to backwash initial filter with a water-only backwash;
(i) closing backwash water valve and dirty backwash water valve of initial filter;
(j) opening influent valve and effluent valve of initial filter;
(j) repeat steps (f) to (j) with each filter to be backwashed;
(l) turning off backwash air blowers for air backwash, closing backwash air valve on final filter;
(m) turning off backwash pumps for water backwash and closing backwash water valve and dirty backwash water valve on final filter;
(n) opening influent valve and effluent valve of final filter.
Alternatively the series of steps to backwash a multi-filter treatment system in a continuous cycle can comprise:
a) Closing the influent and effluent valves, and opening the backwash air valve of the initial filter to be backwashed;
b) Opening the backwash water and dirty backwash water valves of the initial filter;
c) Turning on the backwash blowers, beginning air backwash of initial filter;
d) Turning on backwash water pumps and backwashing initial filter with a combined air/water backwash;
e) closing the influent and effluent valves of the next filter to be backwashed and opening backwash air valve of next filter;
f) closing the backwash air valve of the initial filter thereby stopping air backwash of the initial filter and starting air backwashing of next filter;
g) opening the backwash water and dirty backwash water valves of next filter and backwashing the next filter with an air/water backwash while continuing a water-only backwash of the initial filter;
h) closing the backwash water and dirty backwash water valves of the initial filter;
i) opening the influent valve and effluent valve of the initial filter;
j) repeating steps (e) through (I) in a continuous cycle until all filters are backwashed;
k) turning off backwash blower and closing backwash air valve of final filter;
l) turning off backwash water pump and closing backwash water and dirty backwash water valves of final filter;
m) opening influent valve and effluent valve of final filter so that all filters are back in operation.
Using the embodiments of the method of this invention, the backwash time for each filter can be reduced from approximately 35 minutes per filter to a range of 2 to 20 minutes per filter, preferably 2 to 15 minutes, most preferably, 5 minutes or less per filter, depending on the water treatment needs at the time of backwashing. Preferably, the backwash time per filter during a continuous cycle of backwashing a multi-filter system is approximately 5 minutes per filter.